This invention relates generally to apparatus for simulating a laboratory rock matrix formation for testing downhole logging tools. More specifically, this invention relates to a stacked laminated glass test formation for testing nuclear well logging instrument responses in a laboratory test situation.
Laboratory test formations are essential to accurately define nuclear well logging instrument responses and environmental corrections. Laboratory measurements are used to determine tool reactions to a number of formation parameters and to a variety of environmental conditions. In recent years computer modeling has become an increasingly important addition to laboratory measurements, but laboratory formations are still needed for bench marks and in those situations where modeling does not yet provide sufficient accuracy. Laboratory formations must reliably simulate appropriate petrophysical and environmental characteristics to produce the nuclear instrument response that would be obtained in the real-world environment.
Laboratory test formations have traditionally been constructed from either large blocks of quarried rock or from specially prepared gravel or sand-sized particles in large tanks. Each of these techniques has certain disadvantages. Homogenous quarried formations in each of the desired porosity ranges are difficult, if not impossible, to find, and their pore fluids cannot easily be changed. Gravel tank formations are more readily flushable with different fluids, but very low porosities cannot be produced. Another disadvantage of both quarried rock and gravel tank formations is that a set of several formations with precisely the same porosity and matrix properties cannot be constructed. For example, a set of formations with different hole sizes will generally have slightly different porosities or slightly different thermal neutron absorption cross sections.
Another model formation that has been suggested was made in horizontal layers, consisting entirely of solid material. Once constructed, formation parameters could not be changed. Still another alternative technique is a model formation consisting of a lattice of vertical novaculite (Arkansas stone) rods of rectangular cross section glued together with spaces to simulate porosity. This fluid-filled model was readily flushable and the porosity could be adjusted over a limited range by inserting additional rods into the vertical spaces. However, the model suffered from vertical neutron streaming in the lattice of stone rods which is undesirable in a testing situation.
The present invention presents a new formation constructed from stacks of large sheets of selected glass and offers significant advantages. These advantages include straightforward porosity adjustment, ease of fluid flushing, and totally consistent matrix properties with no vertical neutron streaming in the formation. Various combinations of glass layers and spacers of differing thicknesses can be used in the stack to represent porosities from zero to well beyond the values normally encountered in reservoir formations. This is done with absolutely no change in the moderating, absorbing, or other properties of the simulated matrix. The test formation construction allows a variety of formation and borehole fluids to be used. Thus, formation and environmental parameters can be changed over a wide range to aid in characterizing the response of nuclear logging instruments.